The Trump campaign made the decision to cancel the event at the University of Illinois at Chicago over security concerns, according to police.

Trump told supporters thousands were packed into the arena when "all of a sudden, a planned attack came out of nowhere."

Trump said his supporters are "nice" and "great" people, but protesters he called "professionally-staged wise guys" were determined to cause problems. He said he was worried his backers would have gotten hurt.

"We would've had a problem like you wouldn't have believed," he added

"We made a decision. And I hated to do this, because frankly it wouldn't have been easier to do," he continued. "But I didn't want to see anybody get hurt. You would have had a problem like they haven't seen in a long time. Because we have people that are so amazing."

Video from the rally showed several skirmishes break out on the arena floor. Another protesters waved a banner that read "Trump = Hate" from the upper deck.

"The organized group of people, many of them thugs, who down our First Amendment rights in Chicago, have totally energized America!" Trump wrote on Twitter early Saturday morning.

The billionaire business, speaking in Vandalia, Ohio, turned his attention to challenger and Ohio Gov. John Kasich.

"He's not tough enough. He's not sharp enough," Trump said.

Earlier in the day, Kasich was critical of Trump for creating a "toxic environment" in the presidential race.

More protesters were kicked out of Trump's Ohio stop. About 31 demonstrators were arrested Friday in St. Louis.